The ability to advance our understanding of sudden unexpected death in epilepsy (SUDEP) is critically dependent on the comparison of postmortem brains from SUDEP cases to control brains from patients with epilepsy who died from other causes and patients without epilepsy who died suddenly. We seek to better define the evidence to support or refute leading theories about the causes of SUDEP: the role of brainstem dysfunction and the role of serotonin and adenosine. Specifically, we hypothesize that alterations of interneurons, purinergic and serotonergic systems in vital autonomic brainstem structures are increased in SUDEP compared to controls. We will use advanced 9.4T MRI imaging complemented by quantitative stereological and immunohistochemical techniques to study the neuronal, interneuronal and astrocytic densities in the central autonomic network structures such as the insula, anterior cingulate cortex, and amygdala, with a focus on critical brainstem regions (e.g., cardiorespiratory and median raphe nuclei) and the neurotransmitters serotonin and adenosine. We will leverage the world's largest collection of formalin fixed SUDEP brains, prospective collection of frozen and formalin fixed brains, and tissue from epilepsy surgery cases to study our hypotheses. Our systematic brainstem/autonomic neuropathological analysis will provide the most complete comparative map of the histopathological and biochemical autonomic system abnormalities that are altered in SUDEP. The combined study of human brain tissue and detailed phenotypic data will provide the strongest and most direct link between potential biomarkers and neuropathological studies in living patients with neuropathological findings from SUDEP cases. Our living patients can link ictal and other phenotypic markers (e.g., MRI) to activity in serotonergic (5-HT1A and 5-HT1C receptors, 5-HT transporter) and the enzyme of serotonin synthesis tryptophan hydroxylases) and purinergic (adenosine kinase activity and adenosine receptor binding) systems in their brain tissue. Our SUDEP postmortem studies will link 9.4T images of the entire brain, as well as serotonergic and purinergic activity in cortical, subcortical and brainstem regions. Together, these studies will, for the first time, bridge detailed phenotypic data, cortical and brainstem studies of serotonergic and purinergic activity. Our study will also work with SUTRA 5: Genetics and Epigenetics of SUDEP to study the potential role of nonsynonymous substitutions in serotonergic and purinergic genes, and how these relate to our quantitative stereological and immunohistochemical findings and phenotypic markers.